Public Release: 23-Jan-1997
Model May Explain "Super-Rotation" Of Earth's Core

Geophysicists at The Johns Hopkins University have developed a
model that may help explain recent findings that suggest the
Earth's
solid inner core rotates faster than the rest of the planet.

The model shows how this so-called "super-rotation" of the inner
core could be caused by electromagnetic forces and the fluid
motions of an outer, liquid core, which surrounds the inner solid
core. The outer core is composed primarily of liquid iron, while
the inner core is a ball of solid iron measuring 2,440 kilometers
(1,500 miles) across.

The model could help to provide insights into the mysterious
processes, originating deep inside the Earth, that generate the
planet's magnetic field.

The Johns Hopkins geophysicists developed their model after
researchers at other institutions discovered the apparent
super-rotation. Those scientists inferred the super-rotation by
analyzing past data from seismic waves traveling through the
Earth's core. One team found that the inner core rotates
roughly 1 degree per year faster than the Earth's mantle, the
part of the planet that lies between the core and the crust.
Another team of seismologists concluded that the inner core
rotates
about 3 degrees per year faster than the mantle.

If it were possible to project some location on the Earth's
surface down onto the surface of the inner core and color it with
a red mark, then over the years one would observe the red mark
moving to the east. At a super-rotation rate of 1 degree per
year, the inner core would lap the mantle within 360 years.

Edmund Halley first observed in the late 17th century that the
magnetic field on the Earth's surface tends to drift westward;
the drift is believed to be caused by fluid motions in the
outermost regions of the liquid outer core. With the inner core
seismic data, scientists now have information that the previously
unobservable flow is moving in the opposite direction, to the
east, and at speeds five to 15 times faster than previously
thought.

Three geophysicists at Johns Hopkins have proposed a mechanism
to explain the apparent super-rotation of the inner core. They
have developed a model based primarily on the structure of
convective motions of the fluid in the outer core. The model was
created by graduate students Jonathan Aurnou and Daniel Brito,
working with Professor Peter
Olson, in the
Department of Earth
and Planetary Sciences.

Fluid moving in the southern hemisphere, just below the solid
inner core, and the complementary motion of fluid in the northern
hemisphere, just above the solid core, combine to speed up the
inner core's rotation so that it spins slightly faster than the
rest of the planet.

Olson has compared the motion of the fluid to that of a
hurricane. "Like a hurricane, it creates a very strong vortex,"
he
said.

The fluid flow has been shown in computer models produced in 1995
by Gary Glatzmaier at the Los Alamos National Laboratory and Paul
Roberts at the University of California, Los Angeles.

Another key factor speeding up the inner core's rotation is the
set of strong electromagnetic forces that act to lock the inner
core to the surrounding outer core fluid. This strong
electromagnetic
"coupling" causes the inner core to rotate to the east, closely
following the
motion of the surrounding outer core fluid.

The Hopkins model also is providing insights into another subject
of keen interest: the magnetic field at the planet's core. The
Earth's main geomagnetic field is generated at the core, but
scientists have little direct information about the exact method
by which the magnetic field is generated. Scientists do know that
the flow of the metallic fluid of the outer core reshapes and
alters the Earth's magnetic field in the region around the inner
core. Understanding more about the inner core super-rotation
allows scientists to roughly estimate the state of the magnetic
field deep within the Earth's core.

"If these recent seismic findings stand up to further scrutiny,
then the super-rotation rate of the Earth's inner core may be
used as a `back door' route to understanding the physical state
of the deepest parts of the Earth," Aurnou said.

The Johns Hopkins model was developed after two teams of
scientists discovered the apparent super-rotation. Xiadong Song
and Paul Richards, scientists at the Lamont-Doherty Earth
Observatory, were the researchers who concluded that the inner
core rotates
roughly 1 degree per year faster than the Earth's mantle. Wu-Jei
Su and Adam
Dziewonski at Harvard University and Raymond Jeanloz at the
University of
California, Berkeley, found that the inner core rotates about 3
degrees per
year faster than the mantle.

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